Concerns over the theoretical role of certain chlorofluorocarbons (CFC's) in the depletion of the stratospheric ozone layer have increased interest in developing hydrogen-containing chlorofluorocarbons (HCFC's) to replace the suspect CFC's.
One such alternative HCFC which may be used as a solvent, such as in cleaning electronic circuit boards, as a blowing agent for the manufacture of polymer foams, as an aerosol propellant and the like and which is expected to have little or no effect upon the stratospheric ozone layer is 1,1-dichloro-1-fluoroethane (CH.sub.3 CCl.sub.2 F, HCFC-141b), a compound which has an atmospheric boiling point of about 32.degree. C.
1,1-Dichloro-1-fluoroethane is a known compound which has been prepared by a number of known methods. One such method is by the halogen exchange reaction of 1,1,1-trichloroethane with hydrogen fluoride, usually in the presence of a halogen exchange catalyst. Such a reaction may be represented by Equation I, ##STR1##
While the reaction as shown in Equation I proceeds very readily, there are several disadvantages to the reaction if it were to be used industrially. One such disadvantage is that, for each molecule of 1,1-dichloro-1-fluoroethane produced, one molecule of hydrogen chloride is also generated which must be recovered and disposed of. Another disadvantage is that the trichloromethyl group of 1,1,1-trichloroethane reacts so readily in the halogen exchange reaction that multiple fluorination almost always occurs under normal reaction conditions. Thus, under conditions favorable for the formation of 1,1-dichloro-1-fluoroethane (CH.sub.3 CCl.sub.2 F), 1-chloro-1,1-difluoroethane (CH.sub.3 CClF.sub.2) and 1,1,1-trifluoroethane (CH.sub.3 CF.sub.3) are formed at the expense of CH.sub.3 CCl.sub.2 F, together with the corresponding increase in the formation of hydrogen chloride.
Another reaction to prepare 1,1-dichloro-1-fluoroethane involves hydrogen fluoride addition to 1,1-dichloroethylene (vinylidene chloride) as represented by Equation (II). EQU CH.sub.2 .dbd.CCl.sub.2 +HF.fwdarw.CH.sub.3 CCl.sub.2 F (II).
The reaction as represented by Equation (II) appears to be ideal for the preparation of 1,1-dichloro-1-fluoroethane provided high yields of 1,1-dichloro-1-fluoroethane are achieved, and the vinylidene chloride is effectively consumed. Note that, in this reaction, hydrogen chloride is not formed. High conversion of vinylidene chloride is necessary since both 1,1-dichloro-1-fluoroethane and vinylidene chloride boil at around 32.degree. C., and separation of these two compounds by conventional methods, such as by distillation, is almost impossible.
In U.S. Pat. No. 3,755,477 Firth et al. disclose a vapor-phase reaction of vinylidene chloride with hydrogen fluoride in the presence of a steam-treated chromium oxide catalyst. The disclosure indicates that, at 70.degree.-80.degree. C. reaction temperature, 45 percent of the fluorinated products was 1,1-dichloro-1-fluoroethane while the remainder were more highly fluorinated products, namely, 1-chloro-1,1-difluoroethane and 1,1,1-trifluoroethane. At 90.degree. C.-100.degree. C. reaction temperature, no 1,1-dichloro-1-fluoroethane was reported to be produced.
In U.S. Pat. No. 3,836,479 Paucksch et al. disclose the preparation of a high surface area aluminum fluoride catalyst by admixing boron oxide with alumina, shaping the mixture, and thereafter treating the shaped catalyst with hydrogen fluoride. The thus-prepared catalyst is claimed to be of high activity in catalyzing the hydrogen fluoride addition to double or triple bond compounds. Paucksch et al. teach at Col. 5, lines 35-51, that the claimed catalysts are especially active in hydrogen fluoride addition to olefins which contain one or more fluorine atoms, such as vinyl fluoride, vinylidene fluoride or tetrafluoroethylene, the reaction starting without any external heat and that, at 40.degree.-100.degree. C., 100% conversion to hydrofluorinated compounds is obtained. However, when the olefin contains chlorine or bromine, such as trichloroethylene, 1,1-dichloroethylene, tribromoethylene or 1,1-dibromoethylene, reaction temperatures of between 150.degree. C. to 500.degree. C. are necessary. This teaching is illustrated in Examples 12-15 which show that, while the reaction of hydrogen fluoride with vinyl fluoride, vinylidene fluoride or tetrafluoroethylene proceed with 100% conversion of the olefins at 55.degree.-60.degree. C., the hydrogen fluoride addition to vinyl chloride required a temperature of 175.degree. C. with only 28% conversion of vinyl chloride.
In U.S. Pat. No. 3,803,241 Stolkin et al. disclose a hydrofluorination catalyst prepared by impregnating vacuum-dried alumina with a chromium salt solution and then activating with a stream of hydrogen fluoride at a temperature below 250.degree. C. Preparative reactions are carried out with excess hydrogen fluoride at 140.degree.-400.degree. C. In Example 1 it is shown that, by the use of this catalyst in the vapor-phase reaction of vinylidene chloride and hydrogen fluoride at 198.degree. C., the product obtained was 98.8 volume-percent 1,1,1-trifluoroethane and only 0.2 volume-percent 1,1-dichloro-1-fluoroethane.
In U.S. Pat. No. 3,904,701 Schultz et al. disclose a hydrofluorination catalyst prepared as in the above cited U.S. Pat. No. 3,803,241 with the exception that the alumina, before treatment with hydrogen fluoride, is impregnated with a bismuth salt solution. In Example 1 the claimed catalyst is used in the vapor-phase reaction of hydrogen fluoride with vinylidene chloride, the reaction temperature being 198.degree. C. to 210.degree. C. The products obtained consisted of 99.7 volume-percent CH.sub.3 CF.sub.3, 0.2 volume-percent CH.sub.3 CF.sub.2 Cl and 0.1 volume-percent CH.sub.2 .dbd.CCl.sub.2. The presence of 1,1-dichloro-1-fluoroethane is not mentioned.
In U.S. Pat. No. 4,147,733 Fiske et al. disclose vapor-phase reaction of aqueous hydrogen fluoride with vinylidene chloride in the presence of a metal fluoride catalyst which is an admixture of aluminum fluoride, chromium fluoride and nickel fluoride at 250.degree. C. to 415.degree. C., wherein conversion of vinylidene chloride to fluorinated products is extremely low--2% at 250.degree. C. and 13.5% at 415.degree. C. There is no mention of 1,1-dichloro-1-fluoroethane as one of the products formed.
It is an object of the present invention to provide an improved process for the preparation of 1,1-dichloro-1-fluoroethane. It is a further object of the present invention to provide a process for the preparation of 1,1-dichloro-1-fluoroethane by hydrogen fluoride addition to 1,1-dichloroethylene providing said 1,1-dichloro-1-fluoroethane in high yields and purity.